An internal combustion engine is provided with a spark plug for igniting an air-fuel mixture in a combustion chamber.
The spark plug is for igniting the air-fuel mixture by generating a spark discharge between two electrodes which are separated from each other.
Various proposals have been made concerning shapes and arrangements of electrodes of the spark plug.
For example, a spark plug disclosed in Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2002-324650) includes a center electrode provided with a columnar center electrode side chip and a ground electrode provided and with a columnar ground electrode side chip.
A spark discharge is generated between a distal end surface of the center electrode side chip and a distal end surface of the ground electrode side chip in the spark plug.
Further, the spark plug has a configuration in which a center axis of the ground electrode side chip is inclined with respect to a center axis of the center electrode side chip (and a center axis of the center electrode).
As a result, the distal end surface of the center electrode side chip and the distal end surface of the ground electrode side chip are not parallel to each other, and both are disposed obliquely opposite to each other.
In such a configuration, a wider space between the ground electrode and the center electrode is secured as compared with a configuration in which the ground electrode extends to a position right above the center electrode (that is, a position overlapping with the center axis of the center electrode).
Therefore, it is possible to prevent such a phenomenon that a flame kernel generated in vicinity of the center electrode comes into contact with a surface of the ground electrode and inhibiting a growth of the flame kernel from occurring, thereby sufficient ignition performance can be performed.
In addition, as a result of the ground electrode being shortened, it is also possible to obtain an effect of improving the heat extractability of the ground electrode in the above configuration.
In order to perform the ignition performance of the spark plug stably over a long period of time, a distance between the electrodes, that is, a discharge distance which is a distance between the center electrode side chip and the ground electrode side chip is desirable to be kept constant for a long period of time.
However, since the surface of each chip wears little by little with an impact of spark discharge, the discharge distance gradually increases.
In the configuration disclosed in the above-mentioned Patent Document 1, a portion closest to the ground electrode side chip in the center electrode side chip is a single point at a position nearest to the ground electrode in a circular edge of the distal end surface.
Therefore, this point becomes a starting point of the spark discharge in an initial stage.
In such a configuration, since the starting point of the spark discharge concentrates at the above-mentioned point, the distal end surface of the center electrode side chip rapidly wears out at the above point.
As a result, the discharge distance between the center electrode side chip and the ground electrode side chip spreads at a relatively high speed, and the ignition performance of the spark plug decreases in a short period of time to cause misfire.